But France says it has conclusive proof that “chemical weapons were used and that they were used by Assad’s regime”.
US sources say the substance is likely to have been a mixture of chlorine, sarin, and other chemicals.
Assad attacked Douma because the rebels there refused to be evacuated to northern Syria, despite repeated assaults on the area, which is home to between 80,000 and 150,000 people.
The Violations Documentation Centre, which records alleged violations of international law in Syria, said there were attacks at 4.30pm and 7.30pm.
Shortly after the second attack more than 500 patients, most of them women and children, showed signs of exposure to a chemical agent and were taken to medical facilities.
Their symptoms included “respiratory distress, central cyanosis [blue skin or lips], excessive oral foaming, corneal burns and the emission of a chlorine-like odour”, according to the VDC.